Method for producing seamless pipes

ABSTRACT

The invention relates to a method for producing seamless pipes of heated massive metal blocks, by means of a mandrel (4), which is fastened on a rolling rod (5), in the case of which a coating material is applied onto the inner side of the hollow block (3) during the forming process by means of the influence of the rolling rod (5) from the massive metal block to a hollow block (3), which is created during the forming.

1. FIELD OF THE INVENTION

The invention relates to a method for producing seamless pipes of heatedmassive metal blocks, in particular comprising a cross-rolling mill, inthe case of which the block is driven by means of the rollers, which areset at an angle, and is rolled via an inner tool, which consists of amandrel, which is fastened on a rolling rod so as to be capable of beingdetached, if necessary.

During the rolling, the rolling rod thereby supports itself against amandrel thrust block with its end, which faces away from the mandrel. Amassive and mostly round metal block, which is heated to rolling heat,is pierced and is stretched in the further process to form a seamlesspipe. The hole is hereby created in that the round block is driven bymeans of the rollers, which are set at an angle, and is rolled via amandrel. It is thereby the object of the mandrel to pierce the core zoneof the block, to smooth the inner surface of the created hollow blockand to bring the wall thickness thereof to the desired measure.

Such a method as well as a device for carrying out the method is knownfrom DE 1 96 04 969 C2, for example. This publication deals inparticular with the wear of the forming tools and the necessity to coolthem as well with the impact of the coolant on the rolling stock itself.

When the heated metal block is in contact with the atmospheric oxygen oroxygen from other sources, such as the cooling water, for instance,scale, which must ideally be detached prior to the further forming, butno later than during the forming so as to prevent surface errors at theinner side of the finally created seamless pipe, are created at theinner surface of the hollow block and also in deforming steps, whichfollow the first forming process, if necessary.

After the embodiment of the hollow block and prior to the furtherforming of the hollow block into a seamless pipe, the method, which istypically used for this, provides for the blow-off of already loosenedscale by means of nitrogen or air as well as for the subsequentintroduction of borate-containing powders, such as borax, for example.For the most part, this borax melts on the surface of the hollow block,loosens the scale to the extent that it can be biown out of the interiorof the hollow block reliably and converts the scale into a liquid form.The introduction of the borate-containing powders takes 4 to 10 seconds.Finally, the discharge, which may be necessary, of the scale, which hasbeen softened, liquefied or loosened by the borate-containing powder,requires an additional 1 to 8 seconds.

The methods known from the state of the art thus not only result in anundesired delay of the production process as a whole, but also to aconsiderable discharge of borax and the burn-off thereof into theenvironment, due to the use of typically approximately 2 kg of borax foreach ton of rolling stock. Finally, the temperature of the hollow blockis also lowered in an undesirable manner by this method step, which wasrequired until now.

To overcome the interfering influences of the mill scale downstream fromthe piercing process, JP 63-154207A additionally proposes theintroduction of a lubricant made of graphite into the area between anelongator mandrel and the inner surface of the hollow block. Theformation of scale, however, is not significantly prevented by this.

2. OBJECT OF THE INVENTION

Based on the above-discussed state of the art, it was thus the object ofthe invention to specify a method for producing seamless pipes, which isable to reliably prevent the disadvantages known from the state of theart. In terms of the invention, this object is solved by means of amethod, comprising the features of claim 1. Advantageous embodiments ofthe invention are presented in the dependent claims.

3. SUMMARY OF THE INVENTION

The invention is based on the knowledge that the formation of scale onthe inner surface of the hollow block and, if necessary, also on theinner side of the seamless pipe, which is later created from the hollowblock, can then be prevented reliably when a coating material (so-called“Piercer Shell Inner Surface Treatment Product” or “Product” in short)is applied onto the inner side of the hollow block already during theforming process under the influence of the mandrel on the massive metalblock and during the entire piercing process.

The formation of scale can be slowed down effectively, if not preventedcompletely by means of the preferably complete coating of the innersurface of the hollow block. In terms of the invention, it is madepossible through this to completely do without the step of looseningscale and the discharge thereof from the formed hollow block, ifnecessary, without having to accept disadvantages with reference to thequality of the inner surface of the hollow block.

The use of borate-containing substances and the discharge thereof intothe environment can furthermore be limited to a minimum and can beprevented completely, if necessary. When using borax as a component ofthe coating material, the material usage and consequently also thedischarge thereof into the environment is only 10-20% as compared to theabove-defined standard methods, due to the required quantities, whichare considerably smaller.

The invention is thus geared to reliably prevent the contact of theinner side of the hollow block with oxygen, in particular theatmospheric oxygen. However, in a particularly advantageous alternativeof the method according to the invention, an inert gas, preferablynitrogen, is used to displace the air within the hollow block and/or theseamless pipe. This can take place, for example, in that inert gas isguided into the interior of the hollow block together with the coatingmaterial and via the same lines and openings.

However, an embodiment of the method according to the invention is alsopreferred, in the case of which the inert gas, preferably nitrogen, issupplied via separate lines ad openings, whereby an uncoupling ofnitrogen supply and coating material supply is attained.

Finally, an embodiment is also preferred, in the case of which the inertgas, preferably nitrogen, is supplied together with the coatingmaterial, and the nitrogen is additionally supplied to any location inthe interior of the hollow block, if necessary, via separate linesand/or separate openings.

It is preferred when the coating material is applied onto the inner sideof the hollow block at least almost immediately after the loosening ofthe inner side of the hollow block from the mandrel. The idea of theinvention thus also comprises methods, in the case of which coatingmaterial is already introduced between the mandrel and the hollow block,even before the inner surface of the hollow block lifts itself from themandrel, due to the shape of the mandrel, and causes the advance of theblock against the mandrel. A contact of the oxygen with the inner sideof the hollow block can be completely prevented through this.

However, a method, in the case of which the coating material is onlyapplied after the loosening of the inner side of the hollow block fromthe mandrel, is also preferred. It goes without saying that theapplication of the coating material should take place as soon aspossible in such a case, so that the formation of scale remains limitedto a minimum, which is considered to be acceptable.

For the application of the coating material in the above-specifiedmanner, it is preferred when openings in the mandrel and/or the rollingrod itself are attached such that the coating material can be applied tothe inner side of the hollow block via these openings. A plurality ofopenings, which are arranged across the periphery of the tool,preferably in an equidistant manner, are hereby particularly preferred,so as to secure a complete and preferably even distribution of thecoating material on the inner surface of the hollow block through thisin cooperation with the rotation of mandrel and/or rolling rod relativeto the hollow block.

Only a small number of minimum demands must be made on the coatingmaterial itself. It must be ensured that after the contact with theinner side of the hollow block, this coating material adheres at leastto the extent that a coating is created, through which the formation ofscale is attained at least considerably, preferably by at least 50%,more preferably by at least 80% as compared to the above-definedstandard methods. For this, the formation of a continuous coating filmcomprising a minimum thickness of at least 1 pm is currently consideredto be advantageous.

A method, in the case of which the coating material embodies anair-impermeable cover layer on the inner side of the hollow block aswell as on the inner side of the seamless pipe, is particularlypreferred. It is extremely preferred hereby when the cover layer on theinner side of the hollow block has a thickness of less than 100 μm,particularly preferably of less than 10 m on average. It is ensuredthrough this that the contact of the inner side of the hollow block withthe atmospheric oxygen, which may be present, or other oxygen, whichenters into the process steps, is prevented reliably.

In a preferred embodiment of the method according to the invention, thecoating material is applied onto the inner side of the hollow block inpowder form by means of a carrier gas. Particularly preferably,pipelines, which lead to the opening through the rolling rod andpossibly also through the mandrel, are used for this, so as to reliablyensure the application of the coating material onto the inner side ofthe hollow block through this. It is particularly preferred hereby whenthe mixture of carrier gas and coating material is introduced into theline at a pressure of less than 20 bar, but preferably 1-5 bar, so as toensure a sufficient pressure at the openings through this.

It is particularly preferred when the grain size of at least 90% of thepowder is less than 840 μm, preferably less than 250 μm and morepreferably between 30 and 50 μm. It is ensured through this that noblockages are to be feared within the supply pipes or openings withinthe rolling rod or the mandrel, and that the formation of a continuouscoating film comprising such grain sizes is supported in a particularlyadvantageous manner.

In an alternative and likewise preferred embodiment of the methodaccording to the invention, the application of the coating material,however, takes place in liquid form, preferably as a powder, which isdissolved in water and/or mixed with water. Through this, the supply ofthe coating material onto the inner side of the hollow block through therolling rod and the mandrel is designed so as to be particularly simple.Furthermore, the liquid form of the supply of the coating material alsosupports the formation of the coating film on the inner side of thehollow block in a particularly advantageous manner.

In a particularly preferred embodiment of this alternative of the methodaccording to the invention, the volume fraction of the liquid,preferably of water, is 60-90% in the mixture or solution. It isfurthermore particularly preferred when the coating material is suppliedthrough the lines in liquid form at a pressure of 5-50 bar, morepreferably 10-25 bar.

Provided that it is to contain borax, the coating material eitherconsists of a mixture of borax and Sodium Tripolyphosphate (NaTTP),preferably together with soap and/or mica, or of borax and sodiumsulfates, preferably by adding graphite. The individual, preferredportions of the respective components, in each case specified in percentby weight, are specified in the following table 1 together with theinformation with regard to the effect for the individual components.

As to the mica, this is understood to be silicates, particularly layeredsilicates, having the general chemical formula DG₂,3[T Oio]X2. wherein Dmeans 12-coordinated cations (K,Na,Ca,Ba,Rb,Cs,NH⁴⁺), G means6-coordinated cations (Li,Mg,Fe²⁺,Mn,Zn,AI,Fe³⁺,Cr,V,Ti), T means4-coordinated cations (Si,AI,Fe³⁺,B,Be) and X means anions (OH\F³¹,Cl⁻,0²⁻,S²⁻) .

According to the invention, mica having Sodium and/or Potassium as wellas Calcium and/or Barium and Silicon and/or Aluminium and/or Iron and/orTitanium as the main components are preferred.

TABLE 1 Portion in the No. Component Mixture % Purpose 1 soap  0-10wetting borax 52-80 scale loosening NaTTP 20-40 scale loosening +surface coverage mica  0-20 lubrication 2 graphite  0-35 lubricationborax 25-65 scale loosening sodium sulfates 20-60 wetting + surfacecoverage

In the event that the coating material, however, is to be completelyfree from borate, which is particularly preferred, the mixture for thecoating material consists substantially of Sodium Tripolyphosphate(NaTTP) and Sodium N-metaphosphate, preferrably Phoskadent M®, in whichthe main component consists of Sodium dimetaphospahte, to which graphiteis also added in a particularly advantageous manner. The individualportions for the percent by weight, which are in each case specified forthe components, are specified in the below-specified table 2 togetherwith the effects of the individual components.

TABLE 2 Portion in the Component Mixture % Purpose graphite  0-10lubrication NaTTP 20-50 scale loosening + surface coverage PhoskadentM ® 10-56 scale loosening + coverage

It can be seen through this that the coating material according to theinvention must not necessarily render a lubricating effect, even if thiscan indeed be considered to be advantageous. In particular, thelubricating effect of a suitably composed coating film for subsequentprocess steps, in particular the production of the seamless pipe fromthe hollow block, can be useful.

A method, in the case of which the coating film remains in the hollowblock once it has been applied during the production of the hollowblocks, and reliably prevents the appearance of scale in the entireproduction process for seamless pipes.

4. BRIEF DESCRIPTION OF THE FIGURES

The invention will be defined in detail below with reference to FIG. 1.

FIG. 1 shows a schematic view of a device for the supply of nitrogenthrough the rolling rod and for the supply of coating material throughthe rolling rod. The coating material is applied by means of aPLC-controlled application system using an adjustable metering device.

5. DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a piercing mill, in the case of which a hollow block 3 isdriven between an upper roller 1, which is set at an angle, and a lowerroller 2, which is set at an angle, via a mandrel 4, which is fastenedon a rolling rod 5 so as to be capable of being detached, in a schematicview. The forming of a massive metal block into a hollow block 3 takesplace hereby viewed from left to right in the figure, wherein the hollowblock shell 3 a detaches from the mandrel 4 in the forming process andforms an air gap between the rolling rod 5 and the inner side of thehollow block 6. In terms of the invention, the supply of coatingmaterial takes place from a coating material bunker 9 via a meteringdevice 10 and a supply line 8 for the coating material through therolling rod and the mandrel, if necessary, toward the inner side of thehollow block 6, so as to effect a complete sealing of the inner side ofthe hollow block 6 through this. The powdery coating material is appliedonto the inner side of the hollow block 6 together with nitrogen in acontrolled manner at a pressure of 1-5 bar through the supply line 8 andthe rolling rod 5. The atmospheric oxygen is already displaced almostcompletely from the hollow block 3 by means of the excess of nitrogen,which does not react with the red-hot metal of the hollow block 3 andwhich has been introduced herein through the rolling rod 5 and the innerside of the hollow block 6. If necessary, additional nitrogen can beadded into the interior of the hollow block 3 via further(non-illustrated) supply lines.

1. A method of making seamless pipes of heated massive metal blocks bymeans of a mandrel fastened on a rolling rod, the method comprising thesteps of: applying a coating material onto the inner side of the hollowblock during the forming process by means of the influence of therolling rod from the massive metal block to a hollow block is createdduring forming.
 2. The method according to claim 1, wherein the coatingmaterial is applied onto the inner side of the hollow block at leastalmost immediately after the loosening of the inner side of the hollowblock from the mandrel.
 3. The method according to claim 1, wherein thecoating material is applied onto the inner side of the hollow block viaopenings formed in the mandrel or in the rolling rod.
 4. The methodaccording to claim 1, wherein the coating material embodies a preferablyair-impermeable cover layer on the inner side of the hollow block and onthe inner side of the seamless pipe.
 5. The method according to claim 4,wherein the cover layer on the inner side of the hollow block has athickness of less than 100 μm on average.
 6. The method according toclaim 1, wherein inert gas is guided into the hollow block andpreferably also the seamless pipe during the forming process.
 7. Themethod according to claim 1, wherein the coating material is appliedonto the inner side of the hollow block in powder form by a carrier gas.8. The method according to claim 7, the carrier gas is used with apressure of less than 20 bar.
 9. The method according to claim 7,wherein the grain size of at least 90% of the powder is less than 840μm.
 10. The method according to claim 1, wherein the coating material isapplied to the inner side of the hollow block as powder dissolved inwater or mixed with water.
 11. The method according to claim 10, whereinthe volume fraction of the water is 60-90% of the mixture or solution.12. The method according to claim 10, wherein the coating material issupplied in liquid form at a pressure of 3 to 40 bar.
 13. The methodaccording to claim 1, wherein the coating material is a mixture of (a)borax and Sodium tripolyphosphate together with soap, mica, or (b) boraxand sodium sulfates together with graphite.
 14. The method according toclaim 1, wherein the coating material is a mixture of sodiumtripolyphosphate and sodium N-metaphosphate free from borate togetherwith graphite.